U.S. patent number 6,570,353 [Application Number 09/853,383] was granted by the patent office on 2003-05-27 for system for the electronic commutation of a brushless dc motor.
This patent grant is currently assigned to ebm Werke GmbH & Co. KG. Invention is credited to Thomas Kilian, Jens Krotsch.
United States Patent |
6,570,353 |
Krotsch , et al. |
May 27, 2003 |
System for the electronic commutation of a brushless DC motor
Abstract
The invention relates to a system for the electronic commutation
of a brushless DC motor (1) having three phase windings (u, v, w)
which are electrically displaced by 120.degree., comprising a
semiconductor bridge (4) consisting of six power semiconductors
(S.sub.1 to S.sub.6), which drives the phase windings (u, v, w) for
generating a rotating magnetic stator field, a control unit (6)
which correspondingly drives the power semiconductors (S.sub.1 to
S.sub.6), and a device for detecting the respective rotational
position of a rotor exhibiting a permanent-magnetic magnet wheel,
the device for detecting the rotor position being constructed as
sensor-less evaluating unit (8), for evaluating the voltage induced
by the rotating magnet wheel which can be measured at the winding
terminal (U, V, W) of the motor which is not currently driven. It
is the object of the present invention to achieve a reduction of
running and commutation noises whilst maintaining the inexpensive
and fault-insensitive sensorless rotor position detection. For this
purpose, the control unit (6) drives the power semiconductors
(S.sub.1 to S.sub.6) in twelve switching states, which are
different with respect to the magnetic field direction effected in
each case, by means of a 12-step commutation over one electrical
revolution of the DC motor in dependence on the rotor
positions.
Inventors: |
Krotsch; Jens (Niederstetten,
DE), Kilian; Thomas (Schontal, DE) |
Assignee: |
ebm Werke GmbH & Co. KG
(Mulfingen, DE)
|
Family
ID: |
7641846 |
Appl.
No.: |
09/853,383 |
Filed: |
May 11, 2001 |
Foreign Application Priority Data
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May 12, 2000 [DE] |
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100 23 370 |
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Current U.S.
Class: |
318/400.24;
318/400.32; 318/721; 318/722 |
Current CPC
Class: |
H02P
6/14 (20130101) |
Current International
Class: |
H02P
6/14 (20060101); H02P 006/10 (); H02P 006/18 () |
Field of
Search: |
;318/138,254,439,700,720,721,722,724 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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33 06 642 |
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Sep 1984 |
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DE |
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36 02 227 |
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Jul 1987 |
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DE |
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39 34 139 |
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Apr 1990 |
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DE |
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195 24 557 |
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Jan 1997 |
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DE |
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0 621 681 |
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Oct 1994 |
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EP |
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0 872 948 |
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Oct 1998 |
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EP |
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0 881 761 |
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Dec 1998 |
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EP |
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Primary Examiner: Ro; Bentsu
Attorney, Agent or Firm: Kilpatrick Stockton, LLP
Claims
What is claimed is:
1. System for the electronic commutation of a brushless DC motor
(1) having three phase windings (u, v, w) which are electrically
displaced by 120.degree., comprising a semiconductor bridge (4)
consisting of six power semiconductors (S.sub.1 to S.sub.6), which
drives the phase windings (u, v, w)for generating a rotating
magnetic stator field, a control unit (6) which correspondingly
drives the power semiconductors (S.sub.1 to S.sub.6), and a device
for detecting the respective rotational position of a rotor
exhibiting a permanent-magnetic magnet wheel, the device for
detecting the rotor position being constructed as sensor-less
evaluating unit (8), for evaluating the voltage induced by the
rotating magnet wheel which can be measured at a winding terminal
of the motor which is not currently driven, characterized in that,
the control unit (6) drives the power semiconductors (S.sub.1 to
S.sub.6) by means of a 12-step commutation in twelve different
switching states over one electrical revolution of the DC motor (1)
in dependence on the rotor positions; and in that, due to the
twelve different switching states of the power semiconductors
(S.sub.1 to S.sub.6) the stator field of the motor (1) is
predetermined in twelve excitation states, which are different with
respect to the direction of the magnetic field, via the windings
(u, v, w), wherein the control unit is operative to drive the power
semiconductors occurring in switching states with two driven power
semiconductors, on the one hand, and on the other hand, switching
states with three driven power semiconductors, whereby the
evaluating unit can measure induced voltage at the winding which is
not currently driven, in a continuously alternating manner.
2. System according to claim 1, characterized by an adaptive
start-up commutation control, in which the commutation times are
automatically adapted in dependence on the running characteristic,
or, respectively, the mechanical time constant of the drive
system.
3. System according to claim 1, characterized in that, the
evaluating unit (8) in each case detects and evaluates the voltage
between a motor winding terminal (U, V, W) and a common reference
point (X).
4. System according to claim 3, characterized in that the common
reference point (X) is established in such a manner that the
measured voltages extend in phase to the voltages induced in the
star-connected phase windings by the rotating magnet wheel or in
phase with the fictitious phase voltages resulting when a delta
connection is transformed into an equivalent star connection.
5. System according to claim 3, characterized in that the reference
point (X) is directly derived from a real winding neutral of the
motor.
6. System according to claim 3, characterized in that the reference
point (X) is an external simulation of a winding neutral of the
motor.
7. System according to claim 1, characterized in that the
evaluating unit (8) is operative to detect zero transitions of the
respectively induced internal voltages (E.sub.u E.sub.v E.sub.w)
with respect to their polarities, and generates, in dependence
thereon, binary output signals (.pi..sub.EU, .pi..sub.EV,
.pi..sub.EW) as control input signals for the control unit (6).
8. System according to claim 1, characterized in that the control
unit (6), for setting the speed of the motor (1), in each case
drives one of the power semiconductors (S.sub.1 to S.sub.6) pulsed
in a pulse-width-modulated manner in dependence on a speed setting
signal (S.sub.D) supplied by a control input (6a).
9. System according to claim 8, characterized in that in each state
of revolution, only one of the power semiconductors (S.sub.1 to
S.sub.6) at a maximum is pulsed in a pulse-width-modulated manner,
in each case over a coherent electrical angle of 60.degree..
10. System according to claim 1, characterized by a
speed-dependently changing commutation, the commutation taking
place at least at each second commutation step, in each case
earlier with increasing speed.
11. System according to claim 1, characterized in that in each case
odd-numbered states (.SIGMA..sub.1, .SIGMA..sub.3 . . .
.SIGMA..sub.11), in which two winding terminals are energized and
one winding terminal is open, on the one hand, and on the other
hand, even-numbered states (.SIGMA..sub.2, .SIGMA..sub.4, . . .
.SIGMA..sub.12), in which all winding terminals (U, V and W) are
energized, alternate over the electrical revolution, the length in
time (t.sub.com,) of the even-numbered states being determined
mathematically from respective speed and a predetermined
commutation angle (.phi..sub.com) for the commutation.
12. System according to claim 11, characterized in that the
commutation angle (.phi..sub.com) provided is degressively changed
with speed for the speed-dependent commutation.
13. System according to claim 1, characterized in that, for
starting the motor (1) from standstill, the rotor is first aligned
into a defined rotational position by applying a constant direct
current to the phase windings (u, v, w).
14. System according to claim 1, characterized in that commutation
takes place by means of the rotor positions detected by the
evaluating unit (8) even during the start-up of the motor (1).
15. System according to claim 14, characterized in that, during the
start-up phase (II) of the motor (1), all phase windings (u, v, w)
are temporarily de-energized by the control unit (6) for better
detection of the induced voltage which is still weak due to the
still relatively low speed.
Description
The present invention relates to a system for the electronic
commutation of a brushless DC motor having three phase windings
which are electrically displaced by 120.degree., comprising a
semiconductor bridge of six power semiconductors, which drives the
phase windings for generating a rotating magnetic stator field, a
control unit which correspondingly drives the power semiconductors,
and a device for detecting the respective rotational positions of a
rotor exhibiting a permanent-magnetic magnet wheel, the device for
detecting the rotor position being constructed as sensor-less
evaluating unit, in such a manner that the voltage induced by the
rotating magnet wheel, which can be measured in each case at the
phase winding terminal which is currently not driven, is detected
and evaluated.
For the electronic commutation of collector-less DC motors, the
control unit driving the semiconductor bridge must, in principle,
be supplied with information on the current rotational position of
the permanent-magnetic rotor relative to the stator so that the
respective suitable phase windings can be driven for generating a
torque in the desired direction of rotation to generate by this
means an optimum rotating stator field. In general, the rotor
position is detected by rotor position sensors, especially Hall
sensors by means of the permanent-magnetic rotor field. In many
cases, however, the motor and its associated commutation
electronics must be spatially separated from one another so that
electrical connecting lines are required via which, on the one
hand, current is supplied to the phase windings, and on the other
hand, the signals of the rotor position sensors are transmitted to
the control electronics. The connecting lines and the necessary
connections, e.g. plug-in connections, however, lead to very high
cost expenditure, especially for production (assembly) and material
and in addition also increase the susceptibility to faults.
In contrast, a sensor-less detection of the rotor positions is
provided in systems of the generic type as a result of which (Hall)
sensors and corresponding separate line connections can be omitted.
Instead, the polarities or polarity changes (zero transitions) of
the voltages induced in the motor windings by the
permanent-magnetic rotor field (internal voltage or, respectively,
EMF=electromotive force) are detected via the respective non-driven
currentless winding terminal and evaluated.
Such a sensorless commutation system is known, for example, from EP
0 881 761 A1. In this arrangement, the voltages at the three motor
winding terminals are detected by an EMF detector circuit, and from
this three corresponding binary output signals are generated for
the sensorless rotor position determination. In this manner, six
different combinations of these output signals are generated over
one electrical revolution of the rotor which makes it possible to
determine the rotor position in 60.degree. segments. Each
combination or each rotor position segment, respectively, is
associated with one switching state of the bridge semiconductors
(one semiconductor being pulsed in a pulse-width modulated manner
for setting the speed in the respective switching state). As a
result, there are six different switching states at 60.degree.
intervals over one electrical rotation. In this arrangement, the
stator excitation (stator field) is commutated in six steps within
one electrical rotation so that this is a 6-step commutation.
Similar commutation circuits with 6-step commutation are also
described in documents EP 0 872 948 A1, DE 39 34 139 C2, DE 33 06
642 C2 and DE 36 02 227 A1.
In U.S. Pat. No. 5,491,393, a commutation control for brushless DC
motors is described, this also being a 6-step commutation in
principle because one electrical rotation of the motor is divided
into six basic steps with respect to the changing direction of
stator excitation. Although each of these basic steps is subdivided
into two sections, but between these there is only one change of
the power semiconductor which is in each case pulsed in a
pulse-width modulated manner. In each step, however, only two of
the six semiconductor switching elements of the power bridge
circuit are in each case active so that it is always only two of
three winding terminals which are actively connected to the
positive or negative terminal of the DC source via the switching
elements. For this reason, there can be only six different
directions of stator excitation so that this is clearly a 6-step
commutation in the proper sense.
A prior art which is very similar to U.S. Pat. No. 5,491,393 is
also disclosed in U.S. Pat. No. 5,835,992, according to which six
times two switching states or switch combinations are also provided
but there, too, the direction of the stator excitation is changed
in only six steps (=6-step commutation).
The present invention is based on the object of creating a system
of the generic type initially described, by means of which a
reduction in running and commutation noises is achieved whilst
maintaining the inexpensive and fault-insensitive sensorless rotor
position detection.
According to the invention, this is achieved by the fact that the
control unit drives the power semiconductors in twelve different
switching states by means of a 12-step commutation beyond one
electrical revolution of the DC motor in dependence on the rotor
positions. This 12-step commutation according to the invention is
effected in such a manner that each of these twelve switching
states causes a different excitation state of the stator relating
to the direction of the magnetic field generated in it. For this
purpose, it is provided that, on the one hand, switching states in
which two power semiconductors of the semiconductor bridge are
driven and, on the other hand, switching states in which three
power semiconductors are driven, continuously occur in an
alternating manner. In contrast, the power semiconductors in a
6-step commutation are driven in switching states which exclusively
lead to six different excitation states of the stator. In the
12-step commutation, the stator excitation is in each case
commutated by smaller angular steps than in a 6-step commutation as
a result of which a continuously circulating stator field is
generated.
The invention is initially based on the finding that the
application of a 6-step commutation leads to motor noises, the
"commutation clicking", due to an abrupt change in the stator
excitation during the commutation time. When the electric motor is
used as drive unit for a ventilator or a blower, these commutation
noises are very pronounced and disturbing, especially in the lower
speed range in which the flow noises recede, and cannot, therefore,
be accepted in many applications. According to the invention, by
comparison, a clear reduction of these noises can be achieved
(especially in motors with an external rotor) in that, instead of
only six switching states, twice the number of switching states of
the power semiconductors of the semiconductor bridge are now
provided per electrical revolution, i.e. the stator excitation is
commutated in twelve steps instead of six steps within an
electrical angle of rotation of 360.degree..
Although such a 12-step commutation is known per se, this is,
however, exclusively with separate sensors for detecting the rotor
position. The reason for this is that, among experts, it has
hitherto always been assumed that sensorless construction of a
12-step commutation is not possible because sensorless rotor
position detection always presupposes that in each case one winding
terminal is currentless, that is to say isolated from the DC source
in order to be able to detect induced internal voltage (EMF) at all
with a sensorless evaluating unit. In distinction from the 6-step
commutation, however, this is not generally so with 12-step
commutation because there are areas of winding current overlaps in
which all three winding terminals carry current.
Thus, the present invention is based on the further finding that
12-step commutation is only possible if care has been taken that
polarity changes or zero-transitions of the internal voltage occur
precisely in those intervals--severely shortened in the case of
12-step commutation--in which the corresponding winding terminal is
in each case isolated from the DC source and can therefore be used
for detecting the control-related magnitudes.
In real terms this is preferably achieved by the fact that EMF
evaluating unit in each case detects and evaluates the internal
voltage between a motor winding terminal and a common reference
point. The reference point can here be either the actual neutral
point of the motor, brought out via a line, if the motor windings
are connected in a star connection, or an equivalent neutral point.
It should be noted in this respect that, of course, a motor delta
connection can also be fictitiously transferred into a star
connection. According to the invention, the EMF evaluating unit
accordingly, in practice, detects the respective phase voltage or
"phase EMF" in the star system instead of the phase-to-phase
"conductor EMF" which can be measured between two winding terminals
and which, by comparison, is phase-shifted by 30.degree.. It is due
to this measure, according to the invention, that zero transitions
of the internal voltage can be detected at all because they fall
precisely into the steps in which a winding terminal is in each
case currently currentless over a short range of 30.degree.. In
contrast, detection and evaluation of the voltage which is in each
case between two motor winding terminals would not be suitable for
12-step commutation because, in this arrangement, the zero
transitions of the induced internal voltage would always occur when
all three winding terminals are carrying current, i.e. are
connected to the DC source so that detection would not be possible
at all.
The invention will now be explained in greater detail by way of an
example and with reference to the drawing, in which:
FIG. 1 shows a basic circuit diagram of a commutation system
according to the invention,
FIG. 2 shows a circuit diagram of a preferred embodiment of an EMF
evaluating unit,
FIG. 3 shows different diagrams for explaining the sequences when
using the commutation system according to the invention,
FIG. 4 shows a flowchart of the control sequence for the
commutation system according to the invention,
FIG. 5 shows a corresponding flowchart especially for the starting
process,
FIG. 6 shows diagrams for start and normal operation, and
FIG. 7 shows an enlarged representation of time domain VII in FIG.
6.
As can be seen firstly from FIG. 1, a 3-phase DC motor 1 is driven
by a commutation system according to the invention (commutation
electronics) 2. Of the motor 1, only a stator having three phase
windings u, v, w which are electrically displaced by in each case
120.degree., is indicated; an associated permanent-magnetic rotor
(magnet wheel) is not shown. In the example shown, the phase
windings u, v, w are connected in star connection but, according to
the invention, a delta connection is also easily possible. The
phase windings u, v, w are connected via the phase terminals U, V,
W to a power section constructed as semiconductor bridge 4. The
semiconductor bridge 4 consists of six power semiconductors S.sub.1
to S.sub.6 which, in turn, are driven by a control unit 6 via
control signals .SIGMA..sub.S1 to .SIGMA..sub.S6 in dependence on
the respective rotational positions of the rotor. To detect the
rotor positions, an EMF evaluating unit 8 is provided which is
connected to the phase winding terminals U, V and W in order to
detect the EMF induced in each case in the phase windings u, v, w
by the rotating rotor or, respectively, the "internal voltage", and
to evaluate it with respect to its polarities or zero transitions,
respectively. In dependence on this, the EMF evaluating unit 8
generates corresponding output signals .pi..sub.EU, .pi..sub.EV and
.pi..sub.EW for the control unit 6. To generate a rotating magnetic
stator field, the control unit 6 drives the power semiconductors
S.sub.1 to S.sub.6 in each case cyclically alternating combinations
in that the winding terminals U, V, W are either connected to the
positive or the negative terminal of a DC source 10 or are isolated
from the voltage source 10 in a high-impedance manner.
High-efficiency operation of the motor 1 is achieved--neglecting
the electrical time constant of the motor phase windings, if the
EMF which can be detected between the winding terminals U, V, W has
the same variation and the same phase angle as the voltage provided
at these terminals by the semiconductor bridge 4. To achieve the
appropriate drive, the control unit 6 is supplied with the output
signals .pi..sub.EU, .pi..sub.EV, and .pi..sub.EW of the EMF
evaluating unit 8 which correspond to the polarity of the EMF
induced in the phase windings u, v, w of the motor 1 and,
respectively, reflect the current position of the motor. In
addition, the control unit 6 has a control input 6a via which a
speed adjustment signal S.sub.D for influencing the motor speed can
be supplied.
In the phases in which two of the three phase windings are
connected to the feeding DC source 10 but the third winding
terminal is isolated from the voltage source 10 in a high-impedance
manner, the latter can be used for detecting the EMF induced in
this phase winding (internal voltage). The EMF evaluating unit 8
detects the respective polarity of the phase EMF and generates from
this the three binary output signals .pi..sub.EU, .pi..sub.EV, and
.pi..sub.SW which in each case are allocated to one phase winding,
and which, for example, supply a logical 1 signal with a positive
voltage across the corresponding phase winding and a logical 0
signal with a negative phase voltage. As a result, six different
output combinations, which can be unambiguously associated with a
particular rotor position (in 60.degree. segments) are obtained
over one electrical revolution of the motor 1--in
principle--analogously to a method involving sensors with Hall
sensor circuits arranged in the motor.
FIG. 3 diagrammatically shows at 3a the voltages (EMFs) E.sub.U,
E.sub.V and E.sub.W induced by the rotating magnet wheel in the
individual phase windings. Diagram 3b shows the corresponding
voltages E.sub.U-V, E.sub.V-W and E.sub.W-U which will be detected
in each case between two winding terminals. In addition, Diagram 3b
shows the real superimposed variation of the voltages U.sub.U-V,
U.sub.V-W, and U.sub.W-U applied to the winding terminals and of
the induced voltages. Diagram 3c shows the output signals
.pi..sub.EU, .pi..sub.EV and .pi..sub.EW of the EMF evaluating unit
8 in an idealized manner and as an example of the star-connected
motor.
Although the states of the output signals shown in FIG. 3c in each
case change every 60.degree. el, it is provided, according to the
invention, that the control unit 6 drives the power semiconductors
S.sub.1 to S.sub.6 of the semiconductor bridge 4 by means of a
12-step commutation, i.e. in twelve different switching states,
beyond one electrical revolution in dependence on the output
signals according to FIG. 3c. In this arrangement, states in which
two of the six power semiconductors are driven, on the one hand,
and, on the other hand, those in which three of the six power
semiconductors are driven, alternate. In the switching states with
two active power semiconductors, two of the three winding terminals
(U, V, W) are connected to the DC source 10 and in those with three
active power semiconductors, all winding terminals are connected to
the DC source. Each of these switching states leads to a different
state of excitation of the stator as far as the direction of the
magnetic field generated in it is concerned. In a preferred
embodiment, one of the power semiconductors is pulsed preferably in
a pulse-width modulated manner, for setting the speed in each of
these switching states.
Since a 12-step commutation will necessarily result in phases in
which temporarily all three winding terminals are at a defined
potential and, as a result, no open winding terminal is available
for detecting the EMF, care is taken according to the invention, to
see that the zero transitions of the EMFs which are relevant to the
control fall into the intervals in which in each case one winding
terminal is isolated from the DC source.
According to FIG. 2, this is achieved by a special type of EMF
evaluating unit a which is designed in such a manner that it in
each case detects and evaluates the internal voltage (EMF) between
a motor winding terminal U, V, W and a common reference point X. In
the circuit example shown in FIG. 2, the reference point X is a
resistively simulated neutral point of the motor. The voltages
picked up across winding terminals U, V, W are compared with the
potential present at reference point X by means of comparators U1A,
U1B and U1C, respectively, at the outputs of which the binary
output signals .pi..sub.EU, .pi..sub.EV and .pi..sub.EW are
generated.
As can also be seen from FIG. 3, the control unit 6 derives the
drive signals .SIGMA..sub.S1 to .SIGMA..sub.S6 for the
semiconductor bridge 4 from the output signals .pi..sub.EU,
.pi..sub.EV and .pi..sub.EW of the EMF evaluating unit 8. This
relationship can be seen in Diagrams 3c and 3d and supplementary
3e; 3d illustrating the individual drive signals for the power
semiconductors and 3e showing the phase windings of the motor in
the respective associated excitation states. The twelve different
switching states .SIGMA..sub.1 to .SIGMA..sub.12 of the power
semiconductors according to FIG. 3d produce 12 different excitation
states of the stator (FIG. 3e) with respect to the voltages applied
to the winding terminals of the motor which lead to the magnetic
stator field being built up in twelve different successive
directions. The principle of operation in normal operation will now
be explained starting with the state .SIGMA..sub.1.
In state .SIGMA..sub.1, switches S.sub.3 and S.sub.6 are active.
S.sub.3 is conducting continuously, S.sub.6 is preferably pulsed in
a pulse-width-modulated manner for setting the speed which is
indicated by the part of .SIGMA..sub.S6 signal drawn shaded. Since
only two semiconductor switches are conducting, detection of the
EMF is possible in phase winding u, through which no current is
flowing at the moment, i.e. at the winding terminal U not connected
to the DC source 10. Due to the rotation of the rotor, the EMF
changes its polarity at time t.sub.2 in this phase winding which
results in a level change of the .pi..sub.EU signal. This edge
triggers switching state .SIGMA..sub.2 which, according to the
12-step commutation is placed between two states of the
conventional 6-step commutation for reducing the commutation noise
and the running noises of the motor. In this state, power
semiconductors S.sub.2, S.sub.3 and S.sub.6 are conducting, i.e.
all winding terminals are connected to the DC source 10 which is
why detection of the EMF is no longer possible. For this reason, a
commutation time t.sub.com is calculated from the current speed of
the rotor and a predetermined angle of rotation .phi..sub.com. This
time t.sub.com begins at time t.sub.2 and ends at time t.sub.3 of
the transition to the next switching state .SIGMA..sub.3, in which
S.sub.6 is switched off. The winding terminal W is no longer
connected to the DC source 10 which makes it possible to detect the
EMF in phase w. This sequence is repeated correspondingly with the
next level change of the .pi..sub.EW signal at time t.sub.4 as can
be seen from FIG. 3d.
In FIG. 4, this control sequence described is shown more generally
in the form of a flowchart. Further explanations are not necessary
due to the text components contained in FIG. 4.
According to FIG. 3, the following regular features exist,
according to the invention, with the pulse-width modulation,
provided in a preferred manner, for changing the effective winding
voltage, i.e. for influencing the speed: a) During each commutation
step, one power semiconductor, at a maximum, is pulsed in a
pulse-width modulated manner. b) Each semiconductor is pulsed, in a
pulse-width modulated manner for a coherent electrical angle of
60.degree.. c) During a change from an even-numbered state
(.SIGMA..sub.2, .SIGMA..sub.4 . . . ), in which in each case three
semiconductors are active, to an odd-numbered state (.SIGMA..sub.1,
.SIGMA..sub.3 . . . ) having in each case two active
semiconductors, the semiconductor pulsed in a pulse-width-modulated
manner does not change. During the change from an odd-numbered
state to an even-numbered state, the semiconductor pulsed in a
pulse-width-modulated manner changes. d) The control factor of the
semiconductor pulsed in a pulse-width-modulated manner is
preferably different for even-numbered and odd-numbered states,
and, in particular, smaller with even-numbered states than with
odd-numbered states.
As has already been explained, the sensorless 12-step commutation
is possible because, in the commutation sequence according to the
invention, in connection with the special EMF evaluating unit 8,
for example according to FIG. 2, the EMF always changes polarity in
a winding at a time in which the corresponding winding terminal is
open and, as a result, is available for detecting and evaluating
the voltage induced by the magnet wheel.
In reality, the electrical time constant of the motor winding is
not zero. As a result, there is a frequency- or speed-dependent
phase shift between the winding current and the alternating voltage
present across the windings as a result of which the motor
efficiency drops. To compensate for this, the commutation should
occur at an earlier time with increasing speed.
According to the invention, this is done via the angle
.phi..sub.com, which is adapted to the current speed of the motor
for the odd-numbered states .SIGMA..sub.1, .SIGMA..sub.3,
.SIGMA..sub.5, .SIGMA..sub.7, .SIGMA..sub.9 and .SIGMA..sub.11.
.phi..sub.com becomes smaller with increasing speed as a result of
this the commutation is performed earlier. Such pre-firing is not
possible in the case of the even-numbered states .SIGMA..sub.2,
.SIGMA..sub.4, .SIGMA..sub.4, .SIGMA..sub.6, .SIGMA..sub.8,
.SIGMA..sub.10 and .SIGMA..sub.12 since otherwise the polarity
change of the EMF could no longer be detected.
In an advantageous embodiment of the invention, the angle
.phi..sub.com is adapted with respect to the required motor
performance in dependence on the speed. In general, the highest
possible efficiency is demanded as a result of which the angle
.phi..sub.com is changed degressively with the speed. The
even-numbered states .SIGMA..sub.2, .SIGMA..sub.4, to
.SIGMA..sub.12, in contrast, are provided at the same time as the
EMF polarity changes.
In another advantageous variant, the even-numbered states are
replaced by the previously existing odd-numbered states from a
fixed speed n.sub.lim of the motor. If the speed is less that
n.sub.lim, the sequence .SIGMA..sub.1, .SIGMA..sub.2,
.SIGMA..sub.3, . . . .SIGMA..sub.11, .SIGMA..sub.12, .SIGMA..sub.1
is predetermined by the control unit 6. If, in contrast, the speed
is higher, the sequence is . . . .SIGMA..sub.1, .SIGMA..sub.1,
.SIGMA..sub.3, .SIGMA..sub.3, . . . .SIGMA..sub.11, .SIGMA..sub.11,
.SIGMA..sub.1 . . . . Naturally, this correspondingly also applies
in the reverse order to the other direction of rotation of the
motor.
Previously, it has been assumed that the motor is rotating (normal
operation). When the motor is standing still, however, no EMF is
(yet) induced in the windings as a result of which there is no
information on the position of the rotor. A special method is
therefore preferably used for the sensorless start. In this method,
the 12-step commutation can be advantageously also used for the
starting process of the motor. In particular, the intermediate
steps additionally inserted in the 12-step commutation compared
with the 6-step commutation are used in this case.
According to the prior art, after the rotor has been aligned in a
defined position by applying direct current to the windings, a
sequence of steps is provided for starting the motor without taking
into consideration the signals of the EMF evaluating unit ("open
loop") until a speed has been achieved which is sufficiently high,
i.e. the amplitude of the EMF is high enough for detecting its
polarity. This sequence of steps is specified for a particular
mechanical time constant of the drive system. Difficulties arise
with changing load conditions or different moments of inertia. It
may occur in this case that the rotor cannot follow the sequence of
steps and, therefore, will not start.
For cost reasons, a "bootstrap circuit" is frequently used for
driving the (odd-numbered) power semiconductors S.sub.1, S.sub.3,
S.sub.5, the "upper ones" according to FIG. 1, for supplying
voltage to the associated driver stages. However, this circuit
principle has the disadvantage that the "upper" semiconductor
switch cannot be switched on for an arbitrary length of time, or a
bridge branch cannot be inactive for an arbitrary length of time
since otherwise the voltage across the bootstrap capacitor can drop
to an inadequate value. When motors are slow to start, there may be
difficulties in the drive.
A preferred starting method according to the invention is intended
to eliminate the said disadvantages.
This preferred starting method is based on the finding that the EMF
can be reliably detected even at relatively low speeds as long as
all windings are de-energized. When current is flowing, higher
speeds, i.e. greater amplitudes of the EMF, are required for being
able to reliably evaluate the EMF due to, among other things, the
interference due to the switching processes in the case of
pulse-width-modulation.
FIG. 6 and the enlarged section in FIG. 7 in each case show the
output signals .pi..sub.EU, .pi..sub.EV and .pi..sub.EW of the EMF
evaluating unit 8 and the current variation i in one of the motor
feed lines. The start-up process from standstill consists of the
operating phases I alignment, II starting sequence and III normal
operation. For the alignment in phase I, a direct current is fed
into all phase windings. The rotor then aligns itself to a
predetermined position. Starting from the unambiguous rotor
position which is now known, current is applied to the motor
windings in a suitable manner until a sufficiently high speed is
reached. Then commutation takes place in accordance with the
principle already described above. The special start-up sequence II
will be explained in greater detail with reference to FIG. 7 in the
text which follows.
After the rotor has been aligned, e.g. in that the switching state
.SIGMA..sub.2 is output by the control unit 6 via the control
signals .SIGMA..sub.S1 to .SIGMA..sub.S6 (compare FIG. 3), state
.SIGMA..sub.6 is provided at a time t.sub.1 which results in a
commutation of the stator field as a result of which the rotor
accelerates into a desired direction of rotation. This state is
maintained for the period t.sub..SIGMA.6 until all semiconductors
of the semiconductor bridge 4 are switched off at time t.sub.2. As
a result, the phase windings become de-energized as a result of
which a reliable detection of the change in polarity of the EMF in
phase u becomes possible at time t.sub.3, inspite of the still
relatively low speed, which is signalled by the rising edge of the
signal .pi..sub.EU. After that, the subsequent state .SIGMA..sub.8
is activated for period t.sub..SIGMA.8. The winding, which has
become de-energized after the switch-off at time t.sub.4, allows a
reliable detection of the change in polarity of the EMF in phase w
signalled by the falling edge of signal .pi..sub.EU at time
t.sub.5. This edge initiates the state .SIGMA..sub.10. This
sequence is correspondingly repeated until either a fixed number of
states N.sub..SIGMA. has been output or a fixed speed n.sub.min has
been exceeded. If this is so, transition to normal operation, i.e.
the application of the abovementioned principle of 12-step
commutation takes place.
FIG. 5 additionally shows the sequence of the basic principle of
the starting process described in the general form of a
flowchart.
The method described has the decisive advantage that it is a closed
loop, i.e. the variation of the EMF is always included in the
control sequence from the first acceleration phase after the
alignment of the rotor. In comparison with an open-loop starting
sequence, in which detection of the EMF is not possible, at the
beginning due to the continuous excitation of the winding, a much
better start-up characteristic is achieved according to the
invention. Even if the motor has to start under a greater load,
commutation always takes place at the correct time since the EMF is
also included.
According to the invention, the states predetermined during
starting sequence II (.SIGMA..sub.2, .SIGMA..sub.6, .SIGMA..sub.8,
.SIGMA..sub.10 . . . in the example) are exclusively the
commutation steps inserted for achieving 12-step commutation, in
which all three Winding terminals are connected to the DC source
10, i.e. in each case three power semiconductors are always active.
To limit the winding current, the "upper" (odd-numbered) power
semiconductors of the semiconductor bridge 4 according to FIG. 1
are pulsed in a pulse-width-modulated manner. This method has the
advantage that the charge of the bootstrap capacitors is retained
which is not always the case in solutions according to the prior
art since here a winding terminal can be open for a prolonged
period during the start-up and the bootstrap capacitor belonging to
the relevant branch of the semiconductor bridge can become
discharged. An advantage of no lesser importance lies in the fact
that the beginning of the states predetermined during the starting
sequence II according to the invention coincides with the change in
polarity of the EMF as a result of which a displacement by
.phi..sub.com can be omitted in contrast with the prior art. During
the start, there is no reliable information on the speed of the
motor which is why a calculation of the required delay time
t.sub.com from .phi..sub.com would be critical.
The times t.sub..SIGMA.6, t.sub..SIGMA.8, t.sub..SIGMA.10, . . .
according to FIG. 7 depend on the lowest mechanical load and the
smallest possible moment of inertia of the rotating parts and can
be calculated in a simple manner from the motion equation or be
determined empirically.
In a further advantageous embodiment of this method, the period
.DELTA.t (compare again FIG. 7) after the turn-off of the
semiconductor switches of the bridge 4, is e.g. at time t.sub.2, up
to the detection of the change in polarity of the EMF in phase
winding u at time t.sub.3, measured by the control unit 6. .DELTA.t
is used as a measure of how quickly the motor is accelerating, i.e.
what the mechanical system time constant is. Depending on the
measured time .DELTA.t.sub.i after the state .SIGMA..sub.i, the
period of the subsequent state t.sub..SIGMA.i+1 is adapted in such
a manner that maximum acceleration is achieved. If .DELTA.t.sub.i
is large, the period t.sub..SIGMA.i+1 increases and conversely.
A possible implementation of this principle can look as follows. At
a fixed starting sequence consisting of successive states (e.g.
.SIGMA..sub.2, .SIGMA..sub.6, .SIGMA..sub.8, .SIGMA..sub.10, . . .
), the associated times t.sub..SIGMA.2, t.sub..SIGMA..sub.6,
t.sub..SIGMA.8, t.sub..SIGMA.10, . . . for various mechanical
system time constants are calculated from the motion equation or
empirically determined and stored in the control unit 6. After each
step of the starting sequence, .DELTA.t is checked to see how far
the predetermined period deviates from the optimum one and the
appropriate period for the subsequent state is thus selected from
the stored times. The method thus automatically adapts to the
mechanical system time constant within certain limits. As a result
of this adaptive method, a good starting characteristic is achieved
within a wide range of different load cases and different moments
of inertia.
A self-learning method which adapts to the respective connected
motor during the starting sequence if, for example, the same
electronics are to be operated with different motors, represents a
further improvement of this principle. However, the prerequisite
for such a method is that the load conditions and moments of
inertia of the system do not significantly change from start to
start which can be generally assumed, e.g. in the field of
ventilation. For the start, a predetermined starting sequence as
already described above is again provided. During the start, the
times .DELTA.t.sub.i are continuously detected and the associated
times t.sub..SIGMA.1 are varied by the correction values
.DELTA.t.sub..SIGMA.i in such a manner that .DELTA.t.sub.i will
tend towards zero during the next start-up of the motor. The
correction values .DELTA.t.sub..SIGMA.i are permanently stored in
the control unit 6, for example in an EEPROM. With this method, the
starting characteristic of the motor improves with each start.
After a few start-ups, the optimum start-up for the given load case
or, respectively, the given moment of inertia is finally reached
and is immediately available for future start-up processes.
Creeping changes in the load case or moment of inertia in the
system are advantageously also compensated for by this procedure as
a result of which a uniformly optimized start-up always becomes
possible.
The control functions described by way of example in the previous
text and forming the basis of the invention are implemented in the
form of combinatorial and sequential logic in the control unit 6,
preferably by means of a microprocessor, microcontroller or a
programmable integrated logic circuit.
Finally, the essential advantages of the invention compared with
the prior art are summarized again as follows.
Inexpensive, sensorless and advantageous method from the point of
view of noise for commutating a three-phase permanently excited
motor in delta or star connection.
Due to the 12-step commutation, the motor currents have close to a
sinusoidal shape. The resultant noise improvements are very
significant, especially in the case of motors with external rotor.
Due to its restricted dynamic range as a result of the large mass
inertia of the external rotor, an external rotor motor is
particularly suitable for the sensorless 12-step commutation
according to the invention. The main reason for this is, according
to the invention, the time t.sub.com must be calculated. These must
be obtained from the period between the preceding zero transition
information of the EMF evaluation. This is possible using simple
means with sufficient accuracy especially if the motor speed does
not change too fast. This makes the external rotor especially
suitable.
Rugged method for the sensorless start by utilizing the
intermediate states additionally inserted in the 12-step
commutation, which are in phase with the changes in polarity of the
EMF.
Measurement of the EMF during the starting process in the
de-energized state of the windings which enables the EMF to be
reliably evaluated even at relatively low speeds. As a result, a
closed loop can also be used during the starting process, i.e. the
information of the EMF evaluation can be included in the start-up
sequence. This improves the starting characteristic and makes it
more tolerant to changes in load and the moment of mass
inertia.
Adaptive starting method by comparing the predetermined commutation
time with the ideal commutation time by detecting the EMF in the
de-energized state of the windings and adaptation of the subsequent
commutation times to the mechanical system time constant.
The invention is not restricted to the exemplary embodiments shown
and described, but also comprises all embodiments working in the
same manner in the sense of the invention as set forth in the
following claims.
* * * * *